1. Field of the Invention
The present invention relates to an optical modulator and a modulation method for multiplexing a plurality of alternating current signals with different frequencies using an electro-optical effect.
2. Description of the Background Art
Recently, with the dissemination of an optical fiber network, there has been an attempt to perform long-distance transmission of modulation signals of a mobile phone, CATV, a wireless LAN device, and the like with optical fiber as well as high-speed data communication with a baseband. An SCM (Sub-Carrier Multiplex) technique is a technique in which a plurality of modulation signals of different modulation formats are frequency-multiplexed and optically transmitted, and signals having a plurality of channels and a plurality of modulation formats can be optically transmitted together similarly to CATV. In addition, recently, researches on optical transmission of signals of a millimeter wave band by the SCM technique have been made.
In the case of performing optical modulation for such purposes, since a wide modulation bandwidth is required, external modulation using a CW (Continuous Wave) light wave is generally used. Particularly, a lithium-niobate (LN) optical modulator is suitable for high-frequency signal transmission since the lithium-niobate (LN) optical modulator is not subject to restrictions on a modulation bandwidth with a dielectric loss.
FIG. 14 illustrates an exemplary configuration of a conventional LN optical modulator 81. Optical waveguides 101 to 104 which are formed on an LN substrate having an electro-optical effect are constructed as a Mach-Zehnder type, and have a modulating electrode 201 and a GND electrode 200. An alternating current signal S and a direct current bias V are combined by a bias tee 301, and applied to the modulating electrode 201. An optical signal which has passed through the optical waveguide 102 is phase-modulated according to the alternating current signal S, and multiplexed with an optical signal which has passes through the optical waveguide 103. An optical signal outputted from the optical wave guide 104 after multiplexing is an optical signal which is intensity-modulated according to the alternating current signal S.
In addition, a conventional LN optical modulator 82 with a different configuration which is disclosed in Japanese Laid-Open Patent Publication 2004-37695 is shown in FIG. 15. The LN optical modulator 82 is configured so that a modulating electrode 202 to which a signal is applied is used instead of the GND electrode 200 of the LN optical modulator 81 shown in FIG. 14. A phase inversion section 401 inverts a phase of the alternating current signal S applied to the electrode 201 by 180 degrees thereby to generate an alternating current signal S′. The generated alternating current signal S′ is combined with a direct current bias V2, and applied to the modulating electrode 202. Such a configuration has merits such as that wavelength chirp can be small, that modulation efficiency for one electrode can be smaller, and the like in comparison to the LN optical modulator 81 shown in FIG. 14.
Meanwhile, in order to frequency-multiplex a plurality of alternating current signals using the conventional LN optical modulator, a multiplexer, a duplexer using a plurality of filters, or the like needs to be used. Also, in order to multiplex in an optical region, two optical modulators need to be connected to a cascade.
However, in the case of performing combination using a multiplexer at a stage of electrical treating, for example, a loss of about 6 dB occurs in a simplest resistance type combination, causing deterioration of CNR. Also, in the case of performing combination using a filter at the stage of electrical treating, a loss is reduced to several tenths of a dB while group delay frequency characteristics of a signal deteriorates. Particularly, in the case of a signal into which a transmission signal is frequency-modulated, the deterioration of the group delay frequency characteristics causes deterioration of distortion characteristics. Thus, a combination method using a filter is not preferable. In addition, a method of multiplexing at the stage of optical treating can obtain excellent characteristics as compared to the combination at the stage of electrical treating, but it needs to use two costly optical modulators and provides a large cost demerit.